I. Recombinant Expression Systems
With the advent of recombinant DNA technology, it has been possible to express foreign proteins in bacteria. However, in order to make commercially practical the use of recombinant DNA technology, it is necessary to obtain a high level expression of the foreign proteins. Initial work in this area fostered the notion that a strong promoter driving expression of a foreign protein on a multicopy expression vector was the only basic requirement for efficient, high level expression (Queen, C., J. Mol. Appl. Genet., 2:1-10 (1983)). Since then, further work has demonstrated that there are many other factors which can have a significant effect on the expression level of a given foreign protein. These factors can be loosely categorized as those dealing either with the genetics of the system or the physiology of the system. The genetics of the system principally involves the promoters, operators, ribosome binding sites and terminators. The physiology of the system involves the fermentation conditions. Each of these factors are discussed in detail below.